Mechanical stimuli are transduced into neural signals by receptor cells of the auditory and vestibular systems, by muscle spindles and tendon organs, and by the myriad types of cutaneous and deep pressure receptors. In no case is the mechanism understood by which a mechanical stimulus generates an electrophysiological response. The studies proposed here are directed at the basic process of mechanical transduction by hair cells, the receptor cells of the auditory and vestibular systems. Hair cells are chosen because the physiological stimulus--deflection of a ciliary bundle--is well defined and can be reproducibly delivered, because much of the necessary background work has been done, and because the auditory and vestibular systems are more critical than other mechanical senses for our interaction with the environment, and more debilitating when damaged or diseased. Three sets of questions will be asked concerning the transduction mechanism. One deals with conformational changes in the membrane channel proteins that underlie transduction, and with the structural proteins that deliver the stimulus to the channel. The second set is directed at characterizing an adaptation process in hair cells that acts to reduce the effective stimulus during mechanical displacements. The third seeks to describe the pore region of the transduction channel in terms of the relative selectivity among permeant ions and the position of binding sites for small organic cations. By using combination of recently developed mechanical and electrophysiological techniques, these questions can begin to be answered at a molecular level. Single hair cells will be voltage clamped with the whole-cell patch-clamp method to record the current through tranduction channels. Their hair bundles will be directly stimulated with step displacements under visual observation, and the bundle motion will be optically measured. Solutions on both sides of the cell membrane will be controlled, and the concentration of drugs of ions that affect the transduction process will be rapidly changed. The understanding of the transduction mechanism gained may help to elucidate the mechanism of noise trauma in the auditory system, and the toxic effects of the aminoglycoside antibiotics on hair cells.